1. Field of the Invention
The invention pertains to an implantable measuring unit for the intracorporal measurement of patient data, particularly of brain (cranial) pressure, for mobile application in everyday conditions, and is an addition to registration DE 196 38 813.9 entitled "Intracorporal Implantable Measuring System".
2. Description of the Relevent Art
In medical applications, measurement probes are introduced into the body, e.g. the head (intracranial), with the aid of a catheter and routed to areas where bio-signals are to be measured. For measurements in the skull, the probes must have a very small diameter--which is why microsensors are preferred--which then are mounted in and bonded with a carrier sleeve.
For example, for the diagnosis of hydrocephalus symptoms, the brain (cranial) pressure is measured intracranially in the hospital's intensive care unit with a probe. Afterwards the probe is pulled out and destroyed, or sterilized in the case of multiple-use probes, and reused on the next patient.
When hydrocephalus, for example, has been diagnosed, a so-called shunt system is routed through, which guides cranial fluid into the abdominal cavity when the cranial pressure rises above an established value in order to avoid excessive pressure in the brain.
Cranial pressure can be measured both in an epidural and a subdural manner. Epidural means that the cranial pressure between the dura mater of the brain and the top of the skull is indirectly determined by the pressure exercised onto the dura mater of the brain by the cranial fluid.
This measurement location provides the advantages that the dura mater of the brain is not penetrated, and therefore an infection of the dura mater is prevented; that the procedure is considerably easier; that no brain tissue is damaged during the measuring process; and that the sensor can remain in its measurement location for an extended period of time.
Subdural measurement means that the sensor is slid under the dura mater of the brain, which must be penetrated for this procedure. Furthermore, the pressure in the brain tissue (parenchyma) can now be measured as well, and often the brain tissue is penetrated in order to enable measurements in the ventricle (intraventricular).
There are various known intracranial measurement systems. For example, Braun Melsungen AG offers an epidural measurement system by the name of"Epidyn". Here a micro pressure sensor is fastened in a metal housing. The sensor is connected with the cords of a cable through which electric signals can be sent to an extracorporal evaluation unit.
Another epidural system is available from Spiegelberg, where a balloon catheter is slid under the top of the skull. Depending on the cranial pressure which the dura mater of the brain transfers to the balloon, the pressure is routed to the outside via a line and can be measured there extracorporally.
Camino offers an intraventricular cranial pressure measuring system with a beam waveguide where pressure measurements can be conducted according to the reflection measurement procedure via a silicone oxide level which changes its position, and therefore its reflection coefficient, depending on the pressure. The reflecting portion is put into a ratio to the light percentage that is sent; this provides information about the intraventricular pressure. The system offers the advantage during a TUV (German Department of Transportation) approval that no electric streams or tensions occur intracorporally.
In addition, single-use intraventricular and parenchymal "low-cost" cranial pressure sensors are available. Since the spring of 1995, Codman (Johnson & Johnson) has been offering a brain pressure sensor with piezo-resistive technology which is adjusted through a switch with trimming potentiometer in the socket.
The measurement systems mentioned above require in-patient treatment of the patient to conduct the pressure measurements because the feeding tubes are very sensitive. It is desirable, however, to measure and record intracorporal pressures under normal living conditions of the patient at regular intervals.
Furthermore, the patient's freedom of movement is limited by being hooked up to the monitors via the catheter. This makes caring for the patient very labor-intensive even though he or she would otherwise be capable of taking care of himself or herself, both emotionally and physically. In addition, there is the risk of incorrect measurements and equipment failures during movement of the patient.
Particularly for the implantation of a shunt system for cranial fluid drainage, an implantable measurement system for controlling the catheter cross-section and the valve opening pressure would be very desirable.
U.S. Pat. No. 4,519,401 describes a telemetric, intracranial pressure measurement implant which requires no cable connections to recording and evaluation units located extracorporally. For this, the first radio unit transmits the measurement signals of a pressure and temperature sensor to a second radio unit. The first radio unit is implanted under the scalp and is connected with the intracorporal sensors. The patient carries the second radio unit with him extracorporally. Both radio units have a sender and a receiver. At established times, the sensors are activated with an impulse which is transmitted from the second radio unit to the first radio unit. The measurement data is then transmitted from the first to the second radio unit where it can be memorized and displayed on a monitor. The above-described system turns the sensors on and off at pre-adjusted intervals. In doing so, however, it is possible that a sudden pressure increase may not be recorded. Additionally, the record density of the measurement data remains constant, independent of the data relevance. It is not possible to obtain a continuous measuring signal because the data intervals of the measurement values are too low. By utilizing radio signals, relatively high transmission output in the vicinity of the brain is required, which could possibly have damaging side effects.
The German disclosure document DE 43 41 903 A1 describes an implantable telemetric endosystem whose outer dimensions are smaller than 1.0 mm.times.1.5 mm.times.0.6 mm. The implantable measuring system has a sensor in connection with a telemetry unit that is coupled inductively to an extracorporal receiving device. The implantable system is supplied inductively with power from the outside so that no batteries have to be implanted. Suggested data transmission procedures include amplitude, frequency and pulse-width modulation. A method for arranging, fastening and wiring the pressure sensor and the telemetry unit is not included in the description.
In "Contactless Inductive-Operation Microcircuits for Medical Applications", by L. Talamonti, G. Porroveccio, and G. Marotta, in IEEE Engineering in Medicine & Biology Society, Proc. of the 10th Annual Intern. Conference, New Orleans, Nov. 4-7, 1988, pages 818-819, an implantable telemetry unit that can be integrated on a chip with pressure and/or temperature sensors is presented. The telemetry unit, however, should be inserted directly beneath the skin for an operation that is free from disruptions and tolerable for the patient. This would make the layout of the pressure/temperature sensor and telemetry unit described not advantageous, however, because the sensor has to be inserted in defined areas of the body, such as in the cranial fluid or under the dura mater of the brain. In practice, this would mean a separation of the sensor from the telemetry unit.
Conventional systems utilize a cable connection between the sensor and the transmission unit, e.g. telemetry unit. The cable connections' realization is very labor and cost-intensive and prone to defects. Additionally, implantation requires great skill on the part of the physician because the cables cannot be slid under the skin and can break and become twisted during the implantation procedure.